Ultrasonic methods are well established in the field of nondestructive material evaluation. Most previous methods are based on bulk ultrasonic waves. Bulk ultrasonic waves, called longitudinal and shear waves, may be used for materials characterization and flaw detection. Longitudinal waves have a particle displacement which is parallel to the direction of propagation. For shear waves, the particle displacement is perpendicular to the direction of propagation. The frequency of these waves is above the audible range. Therefore, they are labeled as ultrasonic waves. For nondestructive evaluation, the ultrasound is in the form of stress waves having amplitudes that are sufficiently small to remain in the elastic regime and not cause any permanent deformation of the material in which the ultrasonic waves are propagating.
The velocities of the bulk ultrasonic waves are well known to be dependent on the elastic properties of the material. The velocity of an ultrasonic wave is determined by the interatomic forces that bond the material together. Thus, the velocity of an ultrasonic wave is directly dependent on the mechanical properties of the material. If there is a change in the mechanical properties, it is detected as a change in the velocity. Therefore, chemical and compositional changes in the material may be detected as changes in the ultrasonic velocity.
Bulk ultrasonic waves are merely one mode of propagation. As indicated by their name, they require a bulk material in which to propagate. The theoretical work that has been performed with these materials typically applies to semi-infinite half-spaces that do not have any geometrical constraints. Another class of ultrasonic waves is surface waves. Ultrasonic surface waves, or so-called "Rayleigh waves," are bound to the surface of the material in which they propagate. They have several unique physical parameters that are used in the present invention for determining the depth profile of material properties. These features include a non-dispersive nature, a penetration depth into the material of approximately one wavelength, and the ability of control of penetration depth by altering wavelength.
When an ultrasonic wave is non-dispersive, its velocity is independent of its frequency. Thus, the velocity of the Rayleigh wave does not change when the frequency of the wave is altered. Using the well known equation in ultrasonics that the velocity is equal to the wavelength multiplied by the frequency, it is clear that if the frequency of a non-dispersive wave changes, the wavelength must also change to maintain the constant velocity. Thus, as the frequency of a Rayleigh wave decreases, the wavelength must increase.
When the wavelength increases, the penetration depth of the Rayleigh wave must also increase. Typically, the penetration depth of a Rayleigh wave in a material is said to be about one wavelength. However, the exact penetration depth is dependent on the properties of the material and may vary between about 1.15 and 1.25 wavelengths in depth. The exact amount of energy that is carried in the final 20 percent of the penetration depth is typically not accounted for as it contains a very small percentage of the overall energy of the Rayleigh wave. The amplitude of the wave decreases by an exponential function, resulting in very little energy in the final penetration depth beyond one wavelength.
The prior art has made use of certain of these unique features of Rayleigh waves to simplify the development of nondestructive materials evaluation methods. As expected, most of these efforts have concentrated on the use of Rayleigh waves to determine near-surface phenomena. It is, for example, known to use the frequency dependent penetration depth characteristics of Rayleigh waves to determine the depth of surface breaking cracks. U.S. Pat. No. 4,274,288 to Tittmann et al. discloses a method for determining the depth of a surface flaw based on generating a Rayleigh surface wave that interacts with the flaw. A sensor is used to determine the acoustic signal reflected from the flaw, which includes one portion of the signal from the surface edge of the flaw and a second portion of the signal from the bottom of the flaw. The depth of the flaw is determined from analyzing the interference between the two portions of the reflected signals in the frequency domain. The signals are converted to the frequency domain by means of a Fourier transform. The crack depth is determined by selecting the frequency for the maximum amplitude of the frequency domain response, which is independent of the angle of detection, and converting this frequency into a crack depth using a formula given in the patent.
U.S. Pat. No. 4,372,163 to Tittmann et al. discloses the use of broadband generation and detection of Rayleigh waves for materials evaluation. The signals are detected at two locations along the travel path of the Rayleigh wave. The signals are converted into the frequency domain using a Fourier transform. The dispersion of the wave in the surface is calculated by utilizing the distance between the first and second locations and the change in phase of the frequency components of the detected waves between the first and second locations. The calculated dispersion data is inverted to derive a subsurface profile of the physical structure of the object to characterize the surface properties of the object.
U.S. Pat. No. 4,765,750 to Wadley discloses the use of narrow band electromagnetic acoustic transducers to obtain Rayleigh wave velocities at several distinct frequencies. Once the velocities are obtained at each frequency, the results are plotted to determine the velocity changes as a function of frequency. The velocity changes indicate the presence of changes in the material properties at different depths. By performing these measurements, the depth profile of the material properties can be obtained. By this method, changes in temperature as a function of depth may be measured after the material has been cast.
According to one aspect of the present invention, a method for obtaining near-surface characteristics of a material is disclosed. According to the method, a broadband ultrasonic Rayleigh wave including a plurality of components is generated, with a generating system, in the material. The Rayleigh wave is detected with a detection system remote from the generating station. The detected Rayleigh wave is filtered to obtain selected ones of the plurality of components of the detected Rayleigh wave at selected frequencies. Velocities of the selected components of the detected Rayleigh wave are determined at the selected frequencies.
According to another aspect of the present invention, a system for determining near-surface characteristics of a material includes a generator for generating a broadband ultrasonic Rayleigh wave having a plurality of components in the material. The system further includes a detection system for detecting the Rayleigh waves. A filter arrangement is provided for filtering the detected Rayleigh waves to obtain selected ones of the plurality components of the detected Rayleigh waves at selected frequencies, and a processor is provided for determining velocities of the selected components of the detected Rayleigh wave at the selected frequencies.
According to another aspect of the present invention, a method for obtaining near-surface characteristics of a material is disclosed. According to the method, a series of single frequency ultrasonic Rayleigh waves are generated, with a generating system, to provide a plurality of Rayleigh waves in the material. The Rayleigh waves are detected with a detection system remote from the generating station. Velocities of the detected Rayleigh waves at the frequency generated by the generating equipment are determined. A depth profile of one or more characteristics of the material is prepared based on the determined Rayleigh wave velocities.
According to yet another aspect of the present invention, a system for determining near-surface characteristics of a material is disclosed. The system includes a generator for generating a series of single frequency ultrasonic Rayleigh waves in the material. The system also includes a detection system, remote from the generator, for detecting the Rayleigh waves at the frequency generated by the generator. The system also includes processing means for calculating Rayleigh wave velocities based on a time of flight from generation of the Rayleigh waves by the generator to detection of the Rayleigh waves by the detection system and for preparing a depth profile of one or more characteristics of the material based on Rayleigh wave velocities.